Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 36

easily damaged components in a MW wind turbine, the number of rotating parts is
greatly reduced and the turbine structure is considerably simplifi ed. As a result, it
signifi cantly increases the reliability and effi ciency of the wind turbine and reduces
turbine noise and maintenance costs. Since direct drive wind generators operate
at relative low speeds, it reduces the wear and tear of the generator. In order to
identify suitable generator concepts for director drive wind turbines, Bang et al.
have compared various direct drive generator systems and concluded that direct
drive permanent-magnet synchronous machines are more superior in terms of the
energy yield, reliability and maintenance costs [89].
Though the concept of direct drive wind turbine has been proposed for a long
time, the modern direct drive techniques have become available until recent three
decades. Presently, direct drive wind turbines have been manufactured by large
wind OEMs. Siemens installed two innovative 3.6 MW direct drive wind turbines
at a site in west Denmark in 2008. The feasibility of building 10 MW direct-drive
wind turbine was investigated by Polinder et al. [ 90 ].
However, direct drive wind turbines have some disadvantages in terms of the
cost, size, and mass, making them diffi cult in manufacturing, shipping, and install-
ing. Without a gearbox, the rotor diameter in a direct drive generator must be made
larger enough to maintain a relative high rotating speed at the air gap. A lot of struc-
tural material must be added to keep the stator and rotor in place for maintaining the
air gap. Therefore, the direct drive wind turbine has a larger size and a higher
weight. For instance, the Siemens 3.6 MW direct drive wind turbine has a total
weight of 265 tons (nacelle 165 tons and rotor 100 tons), as compared with 235 tons
for a 3.6 MW geared turbine [91]. This requires the higher strength for the turbine
tower. According to Bang et al. [89], the cost of a 3 MW direct drive PM synchro-
nous generator system could be 35% higher than that of a 3 MW induction genera-
tor system with three stage gearbox. To make the direct drive wind turbines more
attractive to the wind market, all these disadvantages must be solved.
7.4 High effi cient blade
Rotor blade design can be split into structural and aerodynamic design. During
normal wind turbine operation, rotor blades have to withstand enormous dynamic
loads. The bending moment due to the gravity load results in up to 10 8 load cycle
alternations within the turbine lifetime. In addition, there are stochastic alternating
loads caused by wind turbulence and the effects of ageing of the materials due to
the weather [92]. As wind turbines become larger and larger, the length, size, and
weight of blades increase accordingly. For instance, the blade diameter in a large
wind turbine could be longer than 100 m, which is higher than the wingspan and
length of Boeing 747-400 at 64.4 and 70.7 m, respectively. There is no doubt that
these blades require extremely high fatigue strength.
In the blade structural design, one indicator is the blade weight/swept area
ratio (or swept area density in some references). It is highly desired to minimize
this ratio while satisfying the blade strength requirements. Most blades used
today are made from composite materials such as glass-fi bre epoxy, carbon